Exposure is used for replicating a pattern of a mask onto a photosensitive material, such as a photoresist formed on an exposed substrate, in which a mask on which the pattern is formed by depositing a shielding material, such as chromium, on a transparent substrate such as glass and then etching it, and short wavelength visible light, ultraviolet rays, deep ultraviolet rays, or the like are then irradiated on the exposed substrate through the mask. A process of selectively leaving only an exposed portion or a non-exposed portion of the photosensitive material, such as development or the like is added after the exposure, so that a pattern of the photosensitive material corresponding to the mask pattern is formed.
The exposure method includes a close contact exposure method, a proximity exposure method, and a projection exposure method.
In the close contact exposure method, the exposure is performed while the mask and the exposed substrate are held in close contact with each other. The reason why the mask and the exposed substrate are closely contacted with each other is to increase resolution. The narrower the space between the mask and the exposed substrate is, the further the influence of diffraction of light emitted from a transparent part of the mask can be reduced, thus increasing the resolution.
However, although it is possible to closely contact the mask with the exposed substrate approximately uniformly in small liquid crystal panels or small-diameter wafers, since it is difficult to make and keep surfaces of the mask and the exposed substrate perfectly plane, and it is also difficult to control so that a contact pressure therebetween may become uniform during the close contact in large-sized exposed substrates, such as a flat display panel or the like, the whole mask cannot be closely contacted uniformly. As a result, force focuses on one portion, and some photosensitive materials where the contact pressure is high easily transfer to the mask. Fragments of the transferred photosensitive material serves as a half-shielding portion or a shielding portion of for mask pattern during the next exposure, thus causing a transcription defect. Additionally, mask defects resulting from the transferred photosensitive materials increase at an accelerated pace by repeating close contact.
Meanwhile, since the uneven contact between the mask and the exposed substrate may cause a relative movement between the mask and the exposed substrate in a plane direction, scratches may be caused in the mask or the exposed substrate if hard contaminants are caught in the contact portion accidentally.
Meanwhile, in the proximity exposure, the exposure is performed by proximately holding the mask and the exposed substrate while providing a small gap (for example, 10 micrometers to 50 micrometers) between the mask and the exposed substrate. Occurrence of a contamination of the mask or a transcription defect due to the photosensitive material on the exposed substrate adhering to the mask side, and occurrence of a scratch on the mask or the exposed substrate due to the exposed substrate and the mask contacting with each other can be considerably reduced further than those of the close contact exposure by employing the proximity exposure.
However, also in the proximity exposure method, when making the large mask and the large exposed substrate proximate to each other, it is difficult to keep the gap between the mask and the exposed substrate uniformly constant. As a result, problems of the close contact exposure such that the mask and the exposed substrate are partially contacted so that some photosensitive materials are transferred to the mask to thereby cause the transcription defect, or the scratch occurs in the mask or the exposed substrate cannot be thoroughly solved. Moreover, the larger an exposure field becomes, the further a risk for the mask and the exposed substrate to partially contact increases.
In addition, when the exposure field is large, a variation in the small gap between the mask and the exposed substrate is also increased, and line width and shape of the pattern replicated by the exposure are dependent on the gap between the mask and the exposed substrate, so that there has been a problem that the variation in the line width and shape of the pattern is increased within the exposure field.
The projection exposure method is a method in which a projection optical system, such as a projection lens optical system in which lenses are arranged in series in combination with each other within a lens-barrel, a projection mirror optical system by a combination of mirrors, a projection optical system by a combination of both lenses and mirrors, or the like is inserted between the mask and the exposed substrate, and a light image of the pattern is formed on the exposed substrate to thereby expose the photosensitive material.
When this method is utilized, the pattern formed on the mask is replicated on the exposed substrate via the projection optical system, and thus the mask and the exposed substrate do not contact with each other, so that problems such that some photosensitive materials are transferred to the mask to thereby cause the transcription defect, or the scratch occurs in the mask or the exposed substrate are not caused.
Additionally, since a working distance and a certain amount of depth of focus can be secured by employing the projection optical system, setting for the exposure can be simplified, so that there is a merit that the variation in the line width and shape of the pattern within the exposure field can be reduced.
However, in this projection exposure method, when a large-sized exposed substrate such as a large-sized glass substrate for flat displays or the like, a plastic substrate, a plastic substrate with copper foil, a screen for screen-printing, a metal sheet, and a large-diameter wafer are exposed, a projection optical system with a large exposure field equal to or larger than the exposed substrate in size is needed if a one-shot exposure is tried. Moreover, it is difficult to manufacture the aforementioned projection lens and projection mirror optical systems with a large diameter, and on top of it, they are significantly expensive since a particular design is required also for maintenance thereof or control of an operating environment, thereby increasing cost of exposure equipment and exposure operation significantly.
Meanwhile, as one measure to cope with the large exposure field, the large field can be exposed by performing a scanning exposure using a small projection optical system and by connecting the exposure fields together. However, a projection optical system which is generally used is a projection optical system to form an inverted real image of the pattern on the mask as represented by the projection lens.
For that reason, when a plurality of projection optical systems described above are arranged next to each other to thereby expose a pattern AB on the mask as illustrated in FIG. 1, a light image A1′B1′ is projected by a projection lens 61, a light image A2′B2′ is projected by a projection lens 62, and a light image A3′B3′ is projected by a projection lens 63, so that the light images are not overlapped at the same position on the exposed substrate, thus not allowing the pattern to be replicated well. Here, a line group 64 indicates light beams.
Hence, in the case of the projection optical system which forms an inverted real image, it has been required that even when the scanning exposure is performed, it is performed by one projection optical system, or a plurality of projection optical systems are arranged sufficiently spaced apart from one another to prevent light from the same pattern from entering into the plurality of projection optical systems simultaneously.
Meanwhile, even when the scanning exposure is performed or the exposure fields are connected together, it is required to precisely synchronize the mask with the exposed substrate to move them in reverse directions, so that there have been problems that a scanning mechanism and its control are complicated and adjustment man-hours of the mechanism increase, maintenance and management of the accuracy take time and effort, or the like.
In order to solve these problems, and to arrange a plurality of lens elements with a small diameter next to one another to thereby form the image of the pattern over the lens elements accurately, it is required that the pattern to be projected is formed as an erected image of the same magnification and the images formed by respective lens elements are overlapped at the same position.
Means for constructing a small, simple and cheap projection optical system to form the erected image of the same magnification includes a projection exposure method of the same magnification in which a gradient refractive index lens array is inserted between the mask and the exposed substrate (for example, Japanese Unexamined Patent Application Publication (Kokai) No. Sho 61-77830).
The gradient refractive index lens array is an optical component in which cylindrical lens elements 71 having a distribution of refractive indexes in a radial direction from a column axis to a circumference surface are arranged in an array shape in a single line or plural lines while circumference surfaces of the cylindrical lens elements contact with each other and they are disposed between base plates 72 for fixing as shown in FIG. 2.
When this gradient refractive index lens array is utilized, an erected real image of the same magnification will be formed if an object surface and an image surface are taken in suitable positions, and a point pattern P and an arrow pattern AB on the mask are overlappingly replicated at the same positions P′ and A′B′ on the exposed substrate by a plurality of adjacent lens elements 65, 66, and 67 of the gradient refractive index lens array as shown in FIG. 3. Here, line groups 68 and 69 indicate light beams.
Hence, if the gradient refractive index lens array is placed between the mask and the exposed substrate to form an erect real image of the same magnification of the mask pattern near the gradient refractive index lens array on the exposed substrate, and the gradient refractive index lens array is used for scanning in a direction perpendicular to an arrangement of the lens elements, a large area can be exposed by the small optical component to thereby replicate the pattern.